1. Field of the Invention
The present invention relates to the pretreatment of plastic materials and articles which are to be metal plated and more particularly to an improvement in the process of surface conditioning such plastic materials and articles so as to provide them with a conducting base or priming coating which permits metal plating of said materials and articles, and products produced thereby.
2. Description of the Prior Art
In recent years electroplating as well as metallic coating of plastics has become of ever increasing importance. In order to provide plastics with firmly bonded metal coatings, the plastic material must be subjected to a pretreatment whereby it is coated with a conductive base coating. Heretofore, such a base coating was produced by carrying out the following process steps:
1. The surface of the plastic material was subjected to a chemical treatment, usually with chromium trioxide containing solutions, which changes the properties of the surface of the plastic material so as to create optimum conditions for the nucleation with and deposition thereon of noble metal ions and to cause satisfactory adherence of the chemically deposited metal layer.
2. The thus pretreated plastic surface is treated with a solution for reducing the chromium trioxide ions.
3. The surface is then activated, usually by immersing it into a solution containing a noble metal ion, such as palladium, silver, gold.
4. The surface is then immersed into a solution of a reducing agent in which the adsorbed noble metal ions are reduced to noble metal nuclei.
When pretreating the plastic surface in the described manner, it is possible to deposit thereon a firmly bonded, continuous metal layer by placing the thus pretreated plastic material into solutions for the electroless metal deposition.
Another way of depositing a metal layer upon the surfaces of plastic materials or articles consists in
1A. CHEMICALLY TREATING THE PLASTIC SURFACE AS DESCRIBED HEREINABOVE UNDER (1),
2A. TREATING THE THUS PRETREATED PLASTIC SURFACE AS DESCRIBED HEREINABOVE UNDER (2),
3A. SENSITIZING THE PLASTIC SURFACE BY IMMERSING IT INTO AN ACIDIFIED SOLUTION OF STANNOUS CHLORIDE, AND
4A. PRODUCING NOBLE METAL NUCLEI THEREON AS DESCRIBED HEREINABOVE UNDER (3).
When proceeding in this manner step (4) as given hereinabove can be omitted.
Usually a mixture of chromium trioxide, sulfuric acid, and water or, respectively, of chromium trioxide, sulfuric acid, phosphoric acid, and water is used in step (1). The composition of such mixtures depends upon the type of plastic to be treated. Mixtures of this type have mainly been used for the pretreatment of ABS (acrylonitrile-butadiene-styrene) plastics and of polypropylene and similar plastics. Duration and temperature of said treatment are dependent not only on the kind of plastic to be treated, but also, when the same kind of plastic is used, on the type supplied by the different manufacturers and, in some instances, on the conditions under which the plastics were manufactured as well as on the geometrical shape of the processed, machined, or molded plastic. In each instance optimum duration and temperature of the treatment are to be predetermined empirically.
The amounts of the components in the chromium trioxide-containing solutions must also be within a predetermined, rather narrow range of concentration. Only deviations of a few percent from the optimum amounts are permissible because otherwise, on subsequent chemical deposition of the metal, the plastic surface is not completely coated by the metal or, respectively, the entire coating does not possess sufficient adhesive strength.
Analytical supervision and dosing of the chemicals consumed during the treatment requires continual control of the concentration of the various components and thus complicates considerably the operating conditions. Such solutions have the further disadvantage that they become useless as soon as they contain a certain content of degradation products of the plastic and of trivalent chromium compounds. To eliminate the used pretreatment solution, it is necessary to reduce the hexavalent chromium compounds whereafter neutralization is required. Thereby, large amounts of highly voluminous chromium hydroxide are formed, the removal of which encumbers very considerably the detoxifying plant. Thus to detoxify the chromium-containing pretreatment baths, requires very considerable expenditures and technical apparatus and plants.
Processes are known to treat the surfaces of plastics with sulfonating agents. Thus antistatic properties are imparted to polystyrene by dipping it into a weak solution of chloro sulfonic acid in an aliphatic hydrocarbon for a short period of time, namely for less than one minute. Such a solution attacks the surface of the polystyrene and adds reactive groups to the polystyrene chain. After rinsing and drying the treated plastic articles, they are dipped into another solution which causes formation of metal salts by ion exchange of the hydrogen atoms of the reactive groups by metal ions. The resulting rinsed and dried articles have imparted thereto satisfactory antistatic properties whereby the transparency of the polystyrene is not affected. This process has been found effective with pure polystyrene only and not with its copolymers and terpolymers, and also not with colored polystyrene. The dilute chlorosulfonic acid solutions in aliphatic hydrocarbons lose their effectiveness in a relatively short period of time, for instance, in up to seven hours.
In another known process polyethylene films and other shaped polyethylene articles are provided with a well adhering and firmly anchored integral coating of a resinous tripolymer of vinylidene chloride, acrylonitrile, and a functionally basic ethenoid monomer such as vinyl pyridine, 2-methyl-5-vinyl pyridine, 2-morpholino ethyl acrylate, N-dimethylamino ethyl acrylate, by first sulfonating the surface of the polyethylene article and subsequently applying a layer of the functionally basic resinous tripolymer to the sulfonated surface. Useful sulfonating reagents are concentrated, at least 98%, sulfuric acid, oleum, anhydrous solutions of oleum and up to about 10% by weight of dissolved sulfur trioxide, or sulfur trioxide vapors. By such a treatment the surface of polyethylene is conditioned so that the layer of the resinous tripolymer subsequently applied to the sulfonated surface is firmly bonded to the polyethylene surface.
It is also known that treatment of most plastics with concentrated sulfuric acid, oleum, or chloro sulfonic acid results in a very considerable roughening and even in decomposition and carbonization or charring of their surfaces.
It is furthermore known to provide polystyrene with a metallic layer by first treating its surface with a solution of sulfur trioxide in a halogenated hydrocarbon and trimethyl phosphate. Such a treatment results in a softening and conditioning of the polystyrene surface so that the adhesive strength of the metal layer applied thereto by electroless plating followed, if required, by electroplating, is insufficient. The conditioning solution has the disadvantage that it is quite unstable because, on standing, decomposition products are formed very soon by the action of sulfur trioxide upon the solvent. The highly poisonous phosgene is one of such decomposition products.
Another disadvantage of this process is to be seen in the fact that the sulfuric acid produced by reaction of sulfur trioxide with the plastic and by the action of the humidity of the atmosphere separates from the conditioning solution in the form of small droplets which settle on the surface of the plastic and thus render impossible uniform conditioning of the entire surface.
Thus, while the first mentioned methods of treating the surface of some plastics do not deal with subsequent chemical metal coating and plating of plastics, this last mentioned conditioning method does not permit uniform plating of the conditioned plastic surface. And the initially described methods of conditioning the surfaces of plastics with the use of chromium trioxide containing solutions have a number of disadvantages and do not yield fully satisfactory metal deposits.